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NROC61 - Ch19 textbook notes

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University of Toronto Scarborough
Le Boutillier

The Brain Rhythms and Sleep The Electroencephalogram y The electroencephalogram EEG is a measurement that enables us to glimpse the generalized activity of the cerebral cortex y Recording Brain Waves o Recording an EEG is relatively simple o The electrodes are wires taped to the scalp some two dozen electrodes are fixed to standard positions on the head and connected to banks of amplifiers and recording devices o Different regions of the brain can be examined o An EEG measures voltages generated by the currents that flow during synaptic excitation of the dendrites of many pyramidal neurons in the cerebral cortex which lies under the skull and makes up 80 of the brains massThe electrical contribution of any single cortical neuron is exceeding small however and must penetrate several layers of nonneural tissue to reach the electrodeSo it takes many thousands of underlying neurons activated together to generate an EEG signal big enough to see all o The amplitude of the EEG signal strongly depends in part on how synchronous the activity of the underlying neurons is o When a group of cells is excited simultaneously the tiny signals sum to generate on large surface signal However when each cell receives the same amount of excitation but spread out in time the summed signals are meagre and irregularThe total amount of excitation may not have changes only the timing of the activity o An alternative way to record the rhythms is withmagnetoencephalography MEGThe capabilities of MEG complement those of other methods that measure brain functionIt is better than EEG at localizing the sources of neural activity in the brainMEG can record rapid fluctuations of neural activity but cannot provide the spatially detailed images of fMRIEEG and MEG directly measure the activity of neurons whereas fMRI and PET detect changes in blood flow or metabolism controlled in part by neuronal activity y EEG Rhythms o EEG rhythms vary dramatically and often correlate with particular states of behaviour and pathology o The rhythms are categorized by their frequency rangeBeta rhythms are the fastestanything greater than about 14Hz and signal an activated cortexAlpha rhythms are about 813 Hz and are associated with quiet waking statesTheta rhythms are about 47 Hz and occurring during some sleep statesDelta rhythms are quite slow less than 4 Hz large in amplitude and the hallmark of deep sleep o EEG will never tell us what a person is thinking but it can help us know if a person is thinking o Lowfrequency high amplitude rhythms are associated with nondreaming sleep states or pathological state of comaThis is logical because when the cortex is actively engaged in processing information the activity level of cortical neurons is relatively high but also relatively unsynchronizedLow synchrony means that the EEG amplitude is low and beta rhythms dominateBy contrast during deep sleep cortical neurons are not engaged in information processing and so synchrony is high and the EEG amplitude is high Sleep y The Functional States of the Brain o Several times during a night you enter a state called rapid eye movement sleep REM sleep
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